Method of driving display device

ABSTRACT

An active matrix type EL display device is provided, which is capable of suppressing the unevenness of luminance display due to the unevenness of the characteristics of TFTs which constitute pixels, or due to variations in the environmental temperature at which the display device is used. The active matrix type EL display is driven by a time gray scale method, and is capable of keeping the drain current of each of its EL driving TFTs constant by operating each of the EL driving TFTs in a saturation region in an ON state. Accordingly, constant current can be made to flow in each of the EL elements, whereby it is possible to provide an active matrix type EL display device with accurate gray scale display and high image quality.

This application is a continuation of U.S. application Ser. No.10/619,881 filed on Jul. 15, 2003 now U.S. Pat. No. 7,158,104 which is acontinuation of U.S. application Ser. No. 09/911,156, filed on Jul. 23,2001 (now U.S. Pat. No. 6,879,110 issued Apr. 12, 2005).

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a method of driving an electronicdisplay device having EL (electro luminescence) elements formed on asubstrate. More particularly, the invention relates to a method ofdriving an EL display device using semiconductor elements (elementsusing semiconductor thin films) as well as to electronic equipment ofthe type in which an EL display device is used as a display part.

Incidentally, the term “EL element” used herein indicates both anelement which uses emission from a singlet exciter (fluorescence) and anelement which uses emission from a triplet exciter (phosphorescence).

2. Description of the Related Art

In recent years, in the field of self-emitting elements, the developmentof EL display devices having EL elements has been becoming more and moreactive. EL display devices are called organic EL displays (OLED(s)) ororganic light emitting diodes (OLED(s)).

Such an EL display device is of the self-emitting type which differsfrom liquid crystal devices. An EL element has a structure in which anEL layer is interposed between a pair of electrodes (an anode and acathode), and ordinary EL layers have a stacked structure.Representatively, there is a stacked structure which is called “holetransport layer/light emitting layer/electron transport layer”, proposedby Tang et al. of Kodak Eastman Company. This structure has very highemission efficiency, and is adopted in nearly all EL display devicescurrently under research and development.

Other structures may also be adopted, such as a structure in which “ahole injection layer, a hole transport layer, a light emitting layer andan electron transport layer” are stacked on an anode in that order, or astructure in which “a hole injection layer, a hole transport layer, alight emitting layer, an electron transport layer and an electroninjection layer” are stacked on an anode in that order. The lightemitting layer may also be doped with a fluorescent pigment or the like.

All the layers provided between a cathode and an anode are hereingenerically called “EL layer”. Accordingly, all the aforementioned holeinjection layer, hole transport layer, light emitting layer, electrontransport layer and electron injection layer are encompassed in the ELlayer.

When a predetermined voltage is applied across a pair of electrodes(both electrodes) of the EL layer with the above-described structure,recombination of carriers occur in the emitting layer, whereby the ELelement emits light. Incidentally, “EL element emits light” is hereincalled “EL element is driven”.

As a driving method for the EL display device, there is an active matrixtype EL display device.

FIG. 3 shows an example of the construction of a pixel portion of anactive matrix type EL display device. A gate signal line (G1 to Gy) towhich a selection signal is to be inputted from a gate signal linedriver circuit is connected to the gate electrode of a switching TFT 301which is provided in each pixel of the pixel portion. Either one of thesource and drain regions of the switching TFT 301 provided in each pixelis connected to a source signal line (S1 to Sx) to which a signal is tobe inputted from a source signal line driver circuit, while the other isconnected to the gate electrode of an EL driving TFT 302 and to eitherone of the electrodes of a capacitor 303 which is provided in eachpixel. The other electrode of the capacitor 303 is connected to a powersupply line (V1 to Vx). Either one of the source and drain regions ofthe EL driving TFT 302 provided in each pixel is connected to the powersupply line (V1 to Vx), while the other is connected to the otherelectrode of the EL element 304 provided in each pixel.

The EL element 304 has an anode, a cathode and an EL layer providedbetween the anode and the cathode. In the case where the anode of the ELelement 304 is connected to the source region or the drain region of theEL driving TFT 302, the anode and the cathode of the EL element 304become a pixel electrode and a counter electrode, respectively.Contrarily, in the case where the cathode of the EL element 304 isconnected to the source region or the drain region of the EL driving TFT302, the cathode and the anode of the EL element 304 become a pixelelectrode and a counter electrode, respectively.

Incidentally, the potential of the counter electrode is herein called“counter potential”, and a power source for applying the counterpotential to the counter electrode is herein called “counter powersource”. The difference between the potential of the pixel electrode andthe potential of the counter electrode is an EL driving voltage, and theEL driving voltage is applied to the EL layer.

As a gray scale display method for the above-described EL displaydevice, there are an analog gray scale method and a time gray scalemethod.

First, the analog gray scale method for the EL display device will bedescribed below. FIG. 4 is a timing chart showing the case where thedisplay device shown in FIG. 3 is driven by the analog gray scalemethod. The period from the moment when one gate signal is selecteduntil the moment when the next gate signal line is selected is hereincalled “one line period (L)”. The period from the moment when one imageis selected until the moment when the next image is selected correspondsto one frame period. In the case of the EL display device shown in FIG.3, since the number of gate signal lines is “y”, y-number of lineperiods (L1 to Ly) are provided in one frame period.

As the resolution of the EL display device becomes higher, the number ofline periods for one frame period becomes larger, and the driver circuitof the EL display device must be driven at a higher frequency.

The power source lines (V1 to Vx) are kept at a constant voltage (powersource potential). In addition, the counter potential is kept constant.The counter potential has a potential difference from the power sourcepotential to such an extent that the EL elements emit light.

In the first line period (L1), a selection signal from the gate signalline driver circuit is inputted to the gate signal line G1. Then, analogvideo signals are inputted to the source signal lines (S1 to Sx) in thisorder.

Since all the switching TFTs 301 connected to the gate signal line G1are turned on, the analog video signals which have been inputted to thesource signal lines (S1 to Sx) are respectively inputted to the ELdriving TFTs 302 via the switching TFTs 301.

According to the potential of the analog video signal inputted to eachof the pixels when the switching TFT 301 is turned on, the gate voltageof the EL driving TFT 302 varies. At this time, the drain current of theEL driving TFT 302 is determined at a 1-to-1 ratio to the gate voltagethereof in accordance with the Id-Vg characteristic of the EL drivingTFT 302. Specifically, according to the potential of the analog videosignals inputted to the gate electrode of the EL driving TFT 302, thepotential of the drain region of the EL driving TFT 302 (an EL drivingvoltage corresponding to the on state of the switching TFT 301) isdetermined and a predetermined drain current flows into the EL element304, and the EL element 304 emits light at the amount of emissioncorresponding to the amount of the drain current.

When the above-described operations are repeated until the terminationof inputting the analog video signals to the respective source signallines (S1 to Sx), the first line period (L1) terminates. Incidentally,one line period may also be defined as the sum of the period requireduntil the termination of inputting the analog video signals to therespective source signal lines (S1 to Sx) and a horizontal retraceperiod. Then, the second line period (L2) starts, and a selection signalis inputted to the gate signal line G2. Similarly to the first lineperiod (L1), analog video signals are inputted to the source signallines (S1 to Sx) in this order.

When selection signals are inputted to all the gate signal lines (G1 toGy), all the line periods (L1 to Ly) terminate. When all the lineperiods (L1 to Ly) terminate, one frame period terminates. During oneframe period, all the pixels perform displaying and one image is formed.Incidentally, one frame period may also be defined as the sum of all theline periods (L1 to Ly) and a vertical retrace period.

As described above, the amounts of emissions of the respective ELelements are controlled by the analog video signals, and gray scaledisplay is provided by the control of the amounts of emissions. In thismanner, in the analog gray scale method, gray scale display is carriedout by the variations in the potentials of the respective analog videosignals inputted to the source signal lines.

The time gray scale method will be described below.

In the time gray scale method, digital signals are inputted to pixels toselect the emitting states or the non-emitting states of the respectiveEL elements, whereby gray scales are represented by the cumulation ofperiods per frame period during which each of the EL elements.

In the following description, 2^(n) gray scales (n is a natural number)are represented. FIG. 5 is a timing chart showing the case where thedisplay device shown in FIG. 3 is driven by the time gray scale method.One frame period is divided into n-number of sub-frame periods (SF₁ toSF_(n)). Incidentally, the period for which all the pixels of the pixelportion displays one image is called “one frame period (F)”. Pluralperiods into which one frame period is divided are called “sub-frameperiods”, respectively. As the number of gray scales increases, thenumber by which one frame period is divided also increases, and thedriver circuit of the EL display device must be driven at a higherfrequency.

One sub-frame period is divided into a write period (Ta) and a displayperiod (Ts). The write period is the period for which digital signalsare inputted to all the pixels during one sub-frame period, and thedisplay period (also called “lighting period”) is the period for whichthe respective EL display devices assume their emitting states ornon-emitting states in accordance with the input digital signals,thereby performing displaying.

The EL driving voltage shown in FIG. 5 represents the EL driving voltageof an EL element for which emitting state is selected. Specifically, theEL driving voltage (FIG. 5) of the EL element for which emitting stateis selected is 0 V during the write period, and has, during the displayperiod, a magnitude which enables the EL element to emit light.

The counter potential is controlled by an external switch (not shown) sothat the counter potential is kept at approximately the same level asthe power source potential during the write period, and has, during thedisplay period, a potential difference from the power source potentialto such an extent that the EL element can emit light.

The write period and the display period of each sub-frame period willfirst be described in detail with reference to FIGS. 3 and 5, andsubsequently, the time gray scale method will be described.

First, a gate signal is inputted to the gate signal line G1, and all theswitching TFTs 301 connected to the gate signal line G1 are turned on.Then, digital signals are inputted to the source signal lines (S1 to Sx)in that order. The counter potential is kept at the same level as thepotential of the power supply lines (V1 to Vx) (power source potential).Each of the digital signals has information of “0” or “1”. Each of thedigital signals of “0” or “1” means a signal which has a voltage of highlevel or low level.

Then, the digital signals which have been inputted to the source signallines (S1 to Sx) are respectively inputted to the gate electrodes of theEL driving TFTs 302 via the switching TFTs 301 which are in the onstate. The respective digital signals are also inputted to thecapacitors 303.

Then, the above-described operations are repeated by inputting gatesignals to the respective gate signal lines (G2 to Gy), whereby digitalsignals are inputted to all the pixels and the input digital signal isheld in each of the pixels. The period required until the digitalsignals are inputted to all the pixels is called “write period”.

When the digital signals are inputted to all the pixels, all theswitching TFTs 301 are turned off. Thus, an external switch (not shown)connected to the counter electrode causes the counter potential to varyso that a potential difference which enables the EL element 304 to emitlight is produced between the counter potential and the power sourcepotential.

In the case where the digital signals have information of “0”, the ELdriving TFTs 302 are turned off and the EL elements 304 do not emitlight. Contrarily, in the case where the digital signals haveinformation of “1”, the EL driving TFTs 302 are turned on. Consequently,the pixel electrodes of the respective EL elements 304 are kept atapproximately the same potential as the power source potential, and theEL elements 304 emit light. In this manner, the emitting states or thenon-emitting states of the EL elements 304 are selected in accordancewith the information of the digital signals, and all the pixels performdisplaying at the same time. When all the pixels perform display, animage is formed. The period for which the pixels perform displaying iscalled “display period”.

The lengths of the write periods (T_(a1) to T_(an)) of all the n-numberof sub-frame periods (SF₁ to SF_(n)) are the same. The display periods(Ts) of the respective sub-frame periods (SF₁ to SF_(n)) are denoted byT_(s1) to T_(sn).

The lengths of the respective display periods are set to becomeT_(S1):T_(S2):T_(S3): . . . :T_(s(n−1)):T_(sn)=2⁰:2⁻¹:2²: . . .:2^(−(n−2)):2^(−(n−1)), respectively. By combining the desired ones ofthese display periods, it is possible to provide display in the desirednumber of gray scales within 2^(n) gray scales.

The display period is any one of T_(s1) to T_(sn). Here, it is assumedthat predetermined pixels are turned on for the period of T_(s1).

Then, when the next write period starts and data signals are inputted toall the pixels, the next display period starts. At this time, thedisplay period is any one of T_(s2) to T_(sn). Here, it is assumed thatpredetermined pixels are turned on for the period of T_(s2).

It is assumed that the same operations are repeated as to the remaining(n−2)-number of sub-frames, whereby the display periods are set asT_(s3), T_(s4), . . . , T_(sn) in this order and predetermined pixelsare turned on during each of the sub-frames.

When the n-number of sub-frame periods appear, one frame periodterminates. At this time, the gray scale of a pixel is determined bycumulatively calculating the length of the display period for which thepixel has been turned on. For example, assuming that n=8 and theobtainable luminance in the case where the pixel emits light for all thedisplay period is 100%, if the pixel emits light during T_(s1) andT_(s2), a luminance of 75% can be represented, and if T_(s3), T_(s5) andT_(s8) are selected, a luminance of 16% can be realized.

Incidentally, in the driving method using the time gray scale methodwhich represents gray scales by inputting n-bit digital signals, thenumber of plural sub-frame periods into which one frame period isdivided, the lengths of the respective sub-frame periods and the likeare not limited to the above-described examples.

The above-described analog gray scale method has problems to bedescribed below.

The analog gray scale method has the problem that the unevenness of thecharacteristics of TFTs greatly affects gray scale display. For example,it is assumed that the Id-Vg characteristics of switching TFTs differbetween two pixels which represent the same gray scale (thecharacteristic of either one of the pixels is shifted as a whole to aplus or minus side relative to the characteristic of the other).

In this case, the drain currents of the respective switching TFTs takedifferent values, and gate voltages with different values are applied tothe EL driving TFTs of the respective pixels. In other words, differentamounts of currents flow into the EL elements of the respective pixels,and as a result, the amounts of emissions from the EL elements differfrom each other and the same gray scale cannot be represented.

Even if equal gate voltages are applied to the EL driving TFTs of therespective pixels, the EL driving TFTs cannot output the same amount ofdrain current so long as the Id-Vg characteristics of the EL drivingTFTs are not even. For this reason, if the Id-Vg characteristics of theswitching TFTs slightly differ from each other, the amounts of currentsoutputted from the EL driving TFTs greatly differ from each other evenwhen equal gate voltages are applied to the EL driving TFTs. As aresult, owing to a slight unevenness of the Id-Vg characteristics, theamounts of emissions from the EL elements greatly differ betweenadjacent pixels even if signals of the same voltage are applied to theEL driving TFTs.

Gray scale display actually becomes far more non-uniform owing to asynergistic effect of the unevenness of the characteristics of theswitching TFTs and the unevenness of the characteristics of the ELdriving TFTs. Thus, analog gray scale display is extremely sensitive tothe unevenness of the characteristics of TFTs. Accordingly, when this ELdisplay device provides gray scale display, there is the problem thatthe display becomes considerably uneven.

The time gray scale method has a problem to be described below.

In the time gray scale method, the luminance of an EL element isrepresented by the time for which a current flows in the EL element andthe EL element emits light. Accordingly, it is possible to greatlysuppress the non-uniformity of display due to the unevenness of thecharacteristics of TFTs, which is a problem in the analog gray scalemethod. However, there is another problem.

The current which flows in the EL element is controlled by a voltage tobe applied across both electrodes of the EL element (EL drivingvoltage). This EL driving voltage is a voltage obtained by subtractingthe voltage across the drain and the source of an EL driving TFT fromthe potential difference between a power source potential and a counterpotential. In order to avoid the influence of the non-uniformity ofdrain-source voltages due to the unevenness of the characteristics of ELdriving TFTs and keep the EL driving voltage constant, the voltageacross the drain and the source of the EL driving TFT is set to be farsmaller than the EL driving voltage. At this time, the EL driving TFT isoperating in a linear region.

In the TFT operation, the linear region corresponds to the operatingregion in which a voltage V_(DS) across the drain and the source of theTFT is smaller than a gate voltage V_(GS) of the TFT.

Here, the current flowing between both electrodes of the EL element isinfluenced by temperature. FIG. 17 is a graph showing the temperaturecharacteristic of the EL element. From this graph, it is possible toknow the amounts of currents which flow between both electrodes of theEL element with respect to voltages applied across both electrodes ofthe EL element at certain temperatures. A temperature T₁ is higher thana temperature T₂, and the temperature T₂ is higher than a temperatureT₃. As can be seen from FIG. 17, even if the voltage applied across theboth electrodes of the EL element in the pixel portion is the same, thecurrent flowing between both electrodes of the EL element becomes largerowing to the temperature characteristic of the EL element as thetemperature of the EL element becomes higher.

The luminance of the EL element is proportional to the amount of currentflowing between both electrodes of the EL element.

In this manner, the time gray scale method has the problem that thecurrent flowing between both electrodes of the EL element varies owingto variations in the environmental temperature at which the EL displaydevice is used if a constant voltage is continuously applied across bothelectrodes of the EL element, and the luminance of the EL display devicevaries and accurate gray scale display becomes impossible.

In the active matrix type EL display device, for the above-describedreasons, if the conventional analog gray scale method or time gray scalemethod is used, it is impossible to perform accurate gray scale display.

SUMMARY OF THE INVENTION

The invention provides a method of driving an EL display device, whichenables accurate gray scale display and hence high-quality imagedisplay.

In accordance with the present invention, an active matrix type ELdisplay device is driven by a time gray scale method. At this time, anEL driving TFT is operated in a saturation region to keep its draincurrent constant with respect to temperature variations.

Accordingly, it is possible to keep constant a current which flowsbetween both electrodes of an EL element, with respect to the unevennessof the characteristics of TFTs and variations in environmentaltemperature, whereby it is possible to provide a method of driving an ELdisplay device which method enables accurate gray scale display andhence high-quality image display.

The construction of the invention will be described below.

In accordance with the present invention, there is provided a method ofdriving a display device which includes pixels each having an EL elementand a transistor, and the method includes the step of dividing one frameperiod into plural sub-frame periods and applying a first gate voltageor a second gate voltage to a gate electrode of the transistor duringeach of the plural sub-frame periods. When the first gate voltage isapplied to the gate electrode of the transistor, a drain current of thetransistor flows across both electrodes of the EL element and the ELelement is placed into an emitting state, and when the second gatevoltage is applied to the gate electrode of the transistor, thetransistor is placed into a non-conductive state and the EL element isplaced into a non-emitting state. An absolute value of the first gatevoltage is not greater than an absolute value of a voltage across adrain and a source of the transistor.

In accordance with the present invention, there is provided a method ofdriving a display device which includes pixels each having an ELelement, a transistor and a resistor, and the method includes the stepof dividing one frame period into plural sub-frame periods and applyinga first gate voltage or a second gate voltage to a gate electrode of thetransistor during each of the plural sub-frame periods. When the firstgate voltage is applied to the gate electrode of the transistor, a draincurrent of the transistor flows across the resistor and both electrodesof the EL element and the EL element is placed into an emitting state,and when the second gate voltage is applied to the gate electrode of thetransistor, the transistor is placed into a non-conductive state and theEL element is placed into a non-emitting state. An absolute value of thefirst gate voltage is not greater than an absolute value of a voltageacross a drain and a source of the transistor.

The method of driving a display device may be a method in which as theratio of a gate width to a gate length of the transistor is smaller than1, the absolute value of the first gate voltage applied to the gateelectrode of the transistor is larger without exceeding the absolutevalue of the voltage across the drain and the source of the transistor.

The method of driving a display device may be a method in which the ELelement enables color display by using an EL layer which emits light ofone color, in combination with a color conversion layer.

The method of driving a display device may be a method in which the ELelement enables color display by using an EL layer which emits whitelight, in combination with a color filter.

The method of driving a display device may be a method in which the ELlayer of the EL element is made of a low molecular weight organicmaterial or a polymeric organic material.

The method of driving a display device may be a method in which the lowmolecular weight organic material is Alq₃(tris-8-quinolinolato-aluminum) or TPD (triphenylamine derivative)

The method of driving a display device may be a method in which thepolymeric organic material is PPV (polyphenylene vinylene), PVK(poly(vinylcarbazole) or polycarbonate.

The method of driving a display device may be a method in which the ELlayer of the EL element is an inorganic material.

The method of driving a display device may be used in a video camera, animage reproducing apparatus, a head-mounted display, a mobile telephoneor a mobile information terminal.

BRIEF DESCRIPTION OF THE DRAWINGS

FIGS. 1A and 1B are views showing a method of driving a display deviceaccording to the invention;

FIG. 2 is a view showing the construction of a pixel portion of adisplay device using the driving method according to the invention;

FIG. 3 is a view showing the construction of a pixel portion of an ELdisplay device;

FIG. 4 is a timing chart showing a method of driving a related art ELdisplay device;

FIG. 5 is a timing chart showing a method of driving an EL displaydevice;

FIG. 6 is a circuit diagram showing a source signal line driver circuitof the EL display device;

FIG. 7 is a top plan view of a latch of the EL display device;

FIGS. 8A to 8C are views showing the process of fabricating an ELdisplay device;

FIGS. 9A, 9B and 9C are views showing the process of fabricating the ELdisplay device;

FIGS. 10A and 10B are views showing the process of fabricating the ELdisplay device;

FIGS. 11A and 11B are a top plan view and a cross-sectional view of anEL display device;

FIGS. 12A and 12B are a top plan view and a cross-sectional view of anEL display device;

FIG. 13 is a cross-sectional view of a pixel portion of an EL displaydevice;

FIG. 14 is a cross-sectional view of a pixel portion of an EL displaydevice;

FIGS. 15A and 15B are a top plan view and a cross-sectional view of anEL display device;

FIG. 16 is a cross-sectional view of an EL display device;

FIG. 17 is a graph showing the temperature characteristic of an ELelement; and

FIGS. 18A to 18E are views showing examples of electronic equipmentprovided with EL display devices using driving methods according to theinvention.

DESCRIPTION OF THE PREFERRED EMBODIMENTS Embodiment Mode

An embodiment mode of the present invention will be described below indetail with reference to FIGS. 1A and 1B.

FIG. 1A is a circuit diagram showing the construction of a pixel of anEL display device according to the present invention. The gate electrodeof a switching TFT 903 is connected to a gate signal line 906. Eitherone of the source and drain regions of the switching TFT 903 isconnected to a source signal line 905, while the other is connected tothe gate electrode of an EL driving TFT 900 and to a capacitor 904.Either one of the source and drain regions of the EL driving TFT 900 isconnected to a power supply line 902, while the other is connected tothe anode or the cathode of the EL element 901.

Let V_(GS) represent a voltage (gate voltage) applied across the gateand the source of the EL driving TFT 900 from the switching TFT 903. LetV_(DS) represent a voltage (drain-source voltage) applied across thedrain and the source of the EL driving TFT 900, and let I_(D) representa current (drain current) which flows between the drain and the sourceat this time. This drain current I_(D) is inputted to the EL element901. Letting V_(EL) represent a voltage (EL driving voltage) appliedacross both electrodes of the EL element 901, a voltage V_(IN) appliedacross a pixel portion (the counter electrode of the EL element 901) anda power supply line 902 is given as the sum of the drain-source voltageV_(DS) and the EL driving voltage V_(EL).

FIG. 1B is a graph showing the relationship between the drain-sourcevoltage V_(DS) and drain current I_(D). The gate voltage V_(GS) isconstant. In this graph, the region in which the drain current I_(D) hasa one to one correspondence to the drain-source voltage V_(DS) is calleda linear region, which corresponds to the case in which the drain-sourcevoltage V_(DS) is small compared to the gate voltage V_(GS). The regionin which the drain current I_(D) is approximately constant with respectto the drain-source voltage V_(DS) is called a saturation region, whichcorresponds to the case in which the drain-source voltage V_(DS) isgreater than or equal to the gate voltage V_(GS).

In the method of driving the EL display device with the conventionaltime gray scale method, control is executed so that the voltage appliedacross both electrodes of the EL element 901 is made constant. In thiscase, if the drain-source voltage V_(DS) of the EL driving TFT 900fluctuates owing to the unevenness of the characteristic of the TFT 900,the EL driving voltage V_(EL) will be influenced. For this reason, inorder to suppress the influence of such unevenness as greatly aspossible, the drain-source voltage V_(DS) of the EL driving TFT 900 isset smaller than the EL driving voltage V_(EL) so that a major part ofthe voltage V_(IN) inputted to the pixel can be applied across bothelectrodes of the EL element 901. Accordingly, the EL driving TFT 900 ismade to operate in the linear region which corresponds to the case inwhich the drain-source voltage V_(DS) is small compared to the gatevoltage V_(GS).

In the EL display device according to the present invention, thedrain-source voltage V_(DS) of the EL driving TFT 900 is set to the gatevoltage V_(GS) or more, and the EL driving TFT 900 is made to operate inthe saturation region in which the constant drain current I_(D) flowsirrespective of the drain-source voltage V_(DS). Accordingly, a constantcurrent is consistently supplied to the EL element 901 irrespective oftemperature changes.

Numerical examples of the voltages applied to the EL element 901 and theEL driving TFT 900 are as follows.

For example, the threshold voltage of the EL driving TFT 900 is madeapproximately 2V. In the case where the gate voltage V_(GS) of the ELdriving TFT 900 is made 5 V with the emitting state of the EL element901 of the pixel being selected, the voltage between the counterelectrode of the EL element 901 and the power supply line 902 (thedifference between the counter potential and the power source potential)during the display period is made approximately 15 V. At this time, thevoltage V_(EL) across both electrodes of the EL element 901 takes avalue of approximately 5-10 V, and the drain-source voltage V_(DS) ofthe EL driving TFT 900 becomes approximately 5 V or more. At this time,the drain-source voltage V_(DS) of the EL driving TFT 900 becomes thegate voltage V_(GS) or more, and the EL driving TFT 900 operates in thesaturation region.

In this manner, a constant current consistently flows in the EL element901 irrespective of temperature changes, whereby the EL element 901emits light at a constant luminance.

EMBODIMENTS

Embodiments of the invention will be described below.

Embodiment 1

Embodiment 1 relates to the method in the above description of theembodiment mode of the present invention, i.e., the method of operatingthe EL driving TFT in the saturation region to keep constant the draincurrent I_(D) which flows across both electrodes of the EL element, andthe following description of Embodiment 1 is a method of suppressing theinfluence of the unevenness of the characteristics of EL driving TFTs.The following description uses the same reference numerals and used inFIG. 1A as well as newly added ones.

In the case where the EL driving TFT 900 is operated in the saturationregion, the following equation (1) is obtained:I _(D)=α(W/L)(V _(GS) −V _(th))²  (1)

In equation (1), I_(D) is the drain current, V_(GS) is the gate voltage,V_(th) is the threshold voltage, W is the gate width, L is the gatelength, and α is a constant. In this case, since the threshold voltageV_(th) has variations, the drain current I_(D) also has variations.

To suppress these variations, the W/L ratio of the gate width W to thegate length L is made small, while the gate voltage VGs is made large,within the range in which the EL driving TFT 900 operates in thesaturation region. In this manner, it is possible to suppress thevariations of the drain current I_(D) due to the variations of thethreshold voltage V_(th) of the EL driving TFT 900.

For example, it is assumed that the threshold voltage V_(th) takes avalue of 2±0.1 V and has a 5% variation, and that the gate voltageV_(GS) is 3 V when W/L is 8. When the value of the drain current I_(D)at this time is calculated, the resultant value has an about 20%variation.

Here, let I_(O) be the average value of the drain current I_(D). If W/Lis made 0.5, the gate voltage V_(GS) needs to be made about 6 V so thatthe average value I_(O) of the drain current I_(D) is made the same aswhen W/L is 8. According to the calculation of the value of the draincurrent I_(D) for the gate voltage V_(GS) of 6 V, it is possible tosuppress the variation of the value to an about 5% variation.

In this manner, it is desirable to make the value of W/L less than 1,preferably 0.5 or less.

Embodiment 2

Embodiment 2 relates to the method in the above description theembodiment mode of the present invention, i.e., the method of operatingthe EL driving TFT in the saturation region to keep constant the draincurrent I_(D) which flows across both electrodes of the EL element, andthe following description of Embodiment 2 is a method of suppressing theinfluence of the unevenness of the characteristics of EL driving TFTs bya method different from that used in Embodiment 1.

FIG. 2 is a circuit diagram showing the construction of a pixel portionof an EL display device according to Embodiment 2. The pixel portionshown in FIG. 2 is the same in basic structure as that shown in FIG. 1A,and in the following description, the modified portions of theconstruction shown in FIG. 1A are denoted by different referencenumerals.

The gate electrode of the switching TFT 903 is connected to the gatesignal line 906. Either one of the source and drain regions of theswitching TFT 903 is connected to the source signal line 905, while theother is connected to the gate electrode of the EL driving TFT 900 andto either one of the electrodes of the capacitor 904. The otherelectrode of the capacitor 904 is connected to the power supply line902. Either one of the source region and the drain region of the ELdriving TFT 900 is connected to the power supply line 902 via a resistor907, while the other is connected to the anode or the cathode of the ELelement 901.

In the case of the construction of the pixel according to Embodiment 2,the equation (1) shown in Embodiment 1 and the following equation (2)are satisfied at the same time:V=V _(GS) +RI _(D)  (2)

In equation (2), V is a potential difference given between the gateelectrode of the EL driving TFT 900 and the power supply line 902, and Ris the resistance value of the resistor 907.

The gate voltage V_(GS) and the drain current I_(D), in the case wherethe resistor 907 is disposed as shown in FIG. 2, are found fromequations (1) and (2). At this time, the variation of the drain currentI_(D) relative to the variation of the threshold voltage V_(th) iscalculated.

For example, in equations (1) and (2), it is assumed that a is 2×10⁻⁶F/V·s and W/L is 1, and that V_(th) takes a value of 2±0.1 V and has a5% variation.

First, consideration is given to the case where R=0 (the resistor 907 isabsent). If V is 4 V, the gate voltage V_(GS) coincides with V at 4 V.The variation of the drain current I_(D) at this time is about 10%. Atthis time, the average value of the drain current I_(D) is about 8×10⁻⁶A.

Then, consideration is given to the case where R=1×10⁶Ω. To hold theaverage value of the drain current I_(D) at about 8×10⁻⁶ A, V is made 12V. At this time, the variation of the drain current I_(D) relative tothe variation of the threshold voltage V_(th) is suppressed to about 1%.

Then, consideration is given to the case where R=2×10⁶Ω. To hold theaverage value of the drain current I_(D) at about 8×10⁻⁶ A, V is made 20V. At this time, the variation of the drain current I_(D) relative tothe variation of the threshold voltage V_(th) is suppressed to about0.6%.

In this manner, with disposing the resistor 907 and setting itsresistance value large, it is possible to suppress the variation of thedrain current I_(D) relative to the variation of the threshold voltageV_(th).

Embodiment 2 can be freely carried out in combination with Embodiment 1.

Embodiment 3

Note that a description is set forth regarding a step for fabricatingTFTs for driver circuit (a source signal line driver circuit and a gatesignal line driver circuit) provided in the pixel portion of a displaydevice using the driver method of the present invention and peripheryportion of the pixel portion. For the simplicity of the explanation, aCMOS circuit is shown in figures, which is a fundamental structurecircuit for the driver circuit portion.

First, as shown in FIG. 8A, a base film 5002 made of an insulating filmsuch as a silicon oxide film, a silicon nitride film, or a siliconoxynitride film, is formed on a substrate 5001 made of a glass such asbarium borosilicate glass or aluminum borosilicate glass, typically aglass such as Corning Corp. #7059 glass or #1737 glass. For example, alamination film of a silicon oxynitride film 5002 a, manufactured fromSiH₄, NH₃, and N₂O by plasma CVD, and formed having a thickness of 10 to200 nm (preferably between 50 and 100 nm), and a hydrogenated siliconoxynitride film 5002 b, similarly manufactured from SiH₄ and N₂O, andformed having a thickness of 50 to 200 nm (preferably between 100 and150 nm), is formed. A two layer structure is shown for the base film5002 in Embodiment 3, but a single layer film of the insulating film,and a structure in which more than two layers are laminated, may also beformed.

Island shape semiconductor layers 5003 to 5006 are formed by crystallinesemiconductor films made from a semiconductor film having an amorphousstructure, using a laser crystallization method or a known thermalcrystallization method. The thickness of the island shape semiconductorlayers 5003 to 5006 may be formed from 25 to 80 nm (preferably between30 and 60 nm). There are no limitations placed on the materials forforming a crystalline semiconductor film, but it is preferable to formthe crystalline semiconductor films by silicon or a silicon germanium(SiGe) alloy.

A laser such as a pulse oscillation type or continuous light emissiontype excimer laser, a YAG laser, or a YVO₄ laser can be used tofabricate the crystalline semiconductor films by the lasercrystallization method. A method of condensing laser light emitted froma laser oscillator into a linear shape by an optical system and thenirradiating the light to the semiconductor film may be used when thesetypes of lasers are used. The crystallization conditions may be suitablyselected by the operator, but when using the excimer laser, the pulseoscillation frequency is set to 30 Hz, and the laser energy density isset form 100 to 400 mJ/cm² (typically between 200 and 300 mJ/cm²).Further, when using the YAG laser, the second harmonic is used and thepulse oscillation frequency is set from 1 to 10 kHz, and the laserenergy density may be set from 300 to 600 mJ/cm² (typically between 350and 500 mJ/cm²). The laser light condensed into a linear shape with awidth of 100 to 1000 μm, for example 400 μm, is then irradiated over theentire surface of the substrate. This is performed with an overlap ratioof 80 to 98% for the linear laser light.

A gate insulating film 5007 is formed covering the island shapesemiconductor layers 5003 to 5006. The gate insulating film 5007 isformed of an insulating film containing silicon with a thickness of 40to 150 nm by plasma CVD or sputtering. A 120 nm thick silicon oxynitridefilm is formed in Embodiment 3. The gate insulating film is not limitedto this type of silicon oxynitride film, of course, and other insulatingfilms containing silicon may also be used in a single layer or in alamination structure. For example, when using a silicon oxide film, itcan be formed by plasma CVD with a mixture of TEOS (tetraethylorthosilicate) and O₂, at a reaction pressure of 40 Pa, with thesubstrate temperature set from 300 to 400° C., and by discharging at ahigh frequency (13.56 MHZ) electric power density of 0.5 to 0.8 W/cm².Good characteristics as a gate insulating film can be obtained bysubsequently performing thermal annealing, at between 400 and 500° C.,of the silicon oxide film thus manufactured.

A first conductive film 5008 and a second conductive film 5009 are thenformed on the gate insulating film 5007 in order to form gateelectrodes. The first conductive film 5008 is formed of a Ta film with athickness of 50 to 100 nm, and the second conductive film 5009 is formedof a W film having a thickness of 100 to 300 nm, in Embodiment 3.

The Ta film is formed by sputtering, and sputtering of a Ta target isperformed by Ar. If appropriate amounts of Xe and Kr are added to Ar,the internal stress of the Ta film is relaxed, and film peeling can beprevented. The resistivity of an α phase Ta film is about 20 μΩcm, andit can be used in the gate electrode, but the resistivity of a β phaseTa film is about 180 μΩcm and it is unsuitable for the gate electrode.The α phase Ta film can easily be obtained if a tantalum nitride film,which possesses a crystal structure similar to that of α phase Ta, isformed with a thickness of about 10 to 50 nm as a base for a Ta film inorder to form the α phase Ta film.

The W film is formed by sputtering with a W target, which can also beformed by thermal CVD using tungsten hexafluoride (WF₆). Whichever isused, it is necessary to make the film become low resistance in order touse it as the gate electrode, and it is preferable that the resistivityof the W film be made equal to or less than 20 μΩcm. The resistivity canbe lowered by enlarging the crystal grains of the W film, but for casesin which there are many impurity elements such as oxygen within the Wfilm, crystallization is inhibited, thereby the film becomes highresistance. A W target having a purity of 99.9999% is thus used insputtering. In addition, by forming the W film while taking sufficientcare that no impurities from the gas phase are introduced at the time offilm formation, the resistivity of 9 to 20 μΩcm can be achieved.

Note that, although the first conductive film 5008 is a Ta film and thesecond conductive film 5009 is a W film in Embodiment 3, both may alsobe formed from an element selected from the group consisting of Ta, W,Ti, Mo, Al, and Cu, or from an alloy material having one of theseelements as its main constituent, and a chemical compound material.Further, a semiconductor film, typically a polycrystalline silicon filminto which an impurity element such as phosphorus is doped, may also beused. Examples of preferable combinations other than that used inEmbodiment 3 include: forming the first conductive film 5008 by tantalumnitride (TaN) and combining it with the second conductive film 5009formed from a W film; forming the first conductive film 5008 by tantalumnitride (TaN) and combining it with the second conductive film 5009formed from an Al film; and forming the first conductive film 5008 bytantalum nitride (TaN) and combining it with the second conductive film5009 formed from a Cu film.

Then, mask 5010 are formed from resist, and a first etching treatment isperformed in order to form electrodes and wirings. An ICP (inductivelycoupled plasma) etching method is used in Embodiment 3. A gas mixture ofCF₄ and Cl₂ is used as an etching gas, and a plasma is generated byapplying a 500 W RF electric power (13.56 MHZ) to a coil shape electrodeat 1 Pa. A 100 W RF electric power (13.56 MHZ) is also applied to thesubstrate side (test piece stage), effectively applying a negativeself-bias voltage. In case of mixing CF₄ and Cl₂, the W film and the Tafilm are etched to the approximately same level.

Edge portions of the first conductive layer and the second conductivelayer are made into a tapered shape in accordance with the effect of thebias voltage applied to the substrate side under the above etchingconditions by using a suitable resist mask shape. The angle of thetapered portions is from 15 to 45°. The etching time may be increased byapproximately 10 to 20% in order to perform etching without any residueremaining on the gate insulating film. The selectivity of a siliconoxynitride film with respect to a W film is from 2 to 4 (typically 3),and therefore approximately 20 to 50 nm of the exposed surface of thesilicon oxynitride film is etched by this over-etching process. Firstshape conductive layers 5011 to 5016 (first conductive layers 5011 a to5016 a and second conductive layers 5011 b to 5016 b) are thus formed ofthe first conductive layers and the second conductive layers inaccordance with the first etching process. Reference numeral 5007denotes a gate insulating film, and the regions not covered by the firstshape conductive layers 5011 to 5016 are made thinner by etching ofabout 20 to 50 nm.

A first doping process is then performed, and an impurity element whichimparts n-type conductivity is added. Ion doping or ion injection may beperformed for the method of doping. Ion doping is performed under theconditions of a dose amount of from 1×10¹³ to 5×10¹⁴ atoms/cm² and anacceleration voltage of 60 to 100 keV. A periodic table group 15element, typically phosphorus (P) or arsenic (As) is used as theimpurity element which imparts n-type conductivity, and phosphorus (P)is used here. The conductive layers 5011 to 5015 become masks withrespect to the n-type conductivity imparting impurity element in thiscase, and first impurity regions 5017 to 5025 are formed in aself-aligning manner. The impurity element which imparts n-typeconductivity is added to the first impurity regions 5017 to 5025 with aconcentration in the range of 1×10²⁰ to 1×10²¹ atoms/cm³. (FIG. 8B)

A second etching process is performed next without removing a resistmask, as shown in FIG. 8C. The W film is etched selectively using amixture of CF₄, Cl₂, and O₂ as a etching gas. The second shapeconductive layers 5026 to 5031 (first conductive layers 5026 a to 5031 aand second conductive layers 5026 b to 5031 b) are formed by secondetching process. Reference numeral 5007 denotes a gate insulating film,and regions not covered by the second shape conductive layers 5026 to5031 are additionally etched on the order of 20 to 50 nm, formingthinner regions.

The etching reaction of a W film or a Ta film in accordance with a mixedgas of CF₄ and Cl₂ can be estimated from the radicals generated and fromthe ion types and vapor pressures of the reaction products. Comparingthe vapor pressures of fluorides and chlorides of W and Ta, the Wfluoride compound WF₆ is extremely high, and the vapor pressures ofWCl₅, TaF₅, and TaCl₅ are of similar order. Therefore the W film and theTa film are both etched by the CF₄ and Cl₂ gas mixture. However, if asuitable quantity of O₂ is added to this gas mixture, CF₄ and O₂ react,forming CO and F, and a large amount of F radicals or F ions isgenerated. As a result, the etching speed of the W film having a highfluoride vapor pressure is increased. On the other hand, even if Fincreases, the etching speed of Ta does not relatively increase.Further, Ta is easily oxidized compared to W, and therefore the surfaceof Ta is oxidized by the addition of O₂. The etching speed of the Tafilm is further reduced because Ta oxides do not react with fluorine andchlorine. Therefore, it becomes possible to have a difference in etchingspeeds between the W film and the Ta film, and it becomes possible tomake the etching speed of the W film larger than that of the Ta film.

A second doping process is then performed, as shown in FIG. 9A. The doseamount is smaller than that of the first doping process in this case,and an impurity element which imparts n-type conductivity is doped underhigh acceleration voltage conditions. For example, doping performed withthe acceleration voltage set from 70 to 120 keV, and a dose amount of1×10¹³ atoms/cm³, and a new impurity region is formed inside the firstimpurity region is formed inside the first impurity region formed in theisland shape semiconductor layers of FIG. 8B. The second conductivelayers 5026 to 5030 are used as masks with respect to the impurityelement, and doping is performed so as to also add the impurity elementinto regions under the first conductive layers 5026 a to 5030 a. Aconcentration of phosphorus (P) added to third impurity regions 5032 to5036 is provided with a gradual concentration gradient in accordancewith a film thickness of the taper portion of the first conductivelayers 5026 a to 5030 a. Further, in the semiconductor layer overlappingthe taper portion of the first conductive layers 5026 a to 5030 a, froman end portion of the taper portion of the second conductive layertoward an inner side, the impurity concentration is more or lessreduced, however, the concentration stays to be substantially the samedegree.

A third etching process is carried out as shown in FIG. 9B. The thirdetching is carried out by using CHF₆ for an etching gas and using areactive ion etching process (RIE process). The third etching process iscarried out for partially etching a taper portion of the firstconductive layers 5026 a to 5031 a and reducing a region overlapping thesemiconductor layer. By the third etching, there are formed thirdconductive layers 5037 through 5042 (first conductive layers 5037 a to5042 a and second conductive layers 5037 b to 5042 b). Reference numeral5007 denotes a gate insulating film, and regions not covered by thethird shape conductive layers 5037 to 5042 are additionally etched onthe order of 20 to 50 nm, forming thinner regions.

By the third etching, there are formed third impurity regions 5032 a to5036 a overlapping the first conductive layers 5037 a to 5041 a in thirdimpurity regions 5032 to 5036. Second impurity regions 5032 b to 5036 bbetween first impurity region and third impurity region.

Fourth impurity regions 5043 to 5054 added with an impurity elementhaving a conductivity type which is the opposite of the firstconductivity type impurity element, are then formed as shown in FIG. 9Cin the island shape semiconductor layers 5004, 5006 which form p-channelTFTs. The third shaped conductive layers 5038 b to 5041 b is used as amask with respect to the impurity element, and the impurity regions areformed in a self-aligning manner. The island shape semiconductor layers5003, 5005 and wiring portion 5042 which form n-channel TFTs, arecovered over their entire surface areas by resist mask 5200. Phosphorusis added to the impurity regions 5043 to 5054 at a differentconcentration, and ion doping is performed here using diborane (B₂H₆),so that the respective impurity regions have the impurity concentrationof 2×10²⁰ to 2×10²¹ atoms/cm³.

Impurity regions are formed in the respective island shape semiconductorlayers by the above processes. The third shaped conductive layers 5037to 5041 overlapping the island shape semiconductor layers function asgate electrodes. The reference numeral 5042 functions as an island shapesource signal line.

A process of activating the impurity elements added to the respectiveisland shape semiconductor layers is then performed with the aim ofcontrolling conductivity type after removing the resist mask 5200.Thermal annealing using an annealing furnace is performed for thisprocess. In addition, laser annealing and rapid thermal annealing (RTA)can also be applied. Thermal annealing is performed with an oxygenconcentration equal to or less than 1 ppm, preferably equal to or lessthan 0.1 ppm, in a nitrogen atmosphere at 400 to 700° C., typicallybetween 500 and 600° C. Heat treatment is performed for 4 hours at 500°C. in Embodiment 3. However, for cases in which the wiring material usedin the third conductive layers 5037 to 5042 is weak with respect toheat, it is preferable to perform activation after forming an interlayerinsulating film (having silicon as its main constituent) in order toprotect the wirings and the like.

In addition, heat treatment is performed for 1 to 12 hours at 300 to450° C. in an atmosphere containing between 3 and 100% hydrogen,performing hydrogenation of the island shape semiconductor layers. Thisprocess is one of terminating dangling bonds in the island shapesemiconductor layers by hydrogen which is thermally excited. Plasmahydrogenation (using hydrogen excited by a plasma) may also be performedas another means of hydrogenation.

As shown in FIG. 10A, a first interlayer insulating film 5055 is formednext of a silicon oxynitride film having a thickness of 100 to 200 nm. Asecond interlayer insulating film 5056 made of an organic insulatingmaterial is then formed on the first interlayer insulating film 5055.After that, the first interlayer film, the second interlayer 5056 andthe contact hole for the gate insulating film 5007 are formed. The pixelelectrode 5063 which is contact to the connect wiring 5062 is patternedto formed after forming each wirings (including connect wiring andsignal wiring) 5057 to 5062 and 5064.

As the second interlayer insulating film 5056, a film made of organicresin is used, and as the organic resin, polyimide, polyamide, acrylic,BCB (benzocyclobutene) or the like can be used. Especially, since thesecond interlayer insulating film 5056 has rather the meaning offlattening, acrylic excellent in flatness is desirable. In thisembodiment, an acrylic film is formed to such a thickness that steppedportions formed by the TFTs can be adequately flattened. It isappropriate that the thickness is preferably made 1 to 5 μm (mostpreferably 2 to 4 μm).

The formation of the contact holes are performed by dry etching or wetetching. Contact holes reaching the n-type impurity regions 5017, 5018,5021 and 5023 or the p-type impurity regions 5043 to 5054, a contacthole reaching to a wiring 5042, a contact hole reaching electric currentsupply line (not shown), and a contact hole (not shown) reaching a gateelectrode are formed, respectively.

Besides, as the wirings (including connect wiring and signal wiring)5057 to 5062, and 5064, a lamination film of three-layer structure isused, in which a Ti film with a thickness of 100 nm, an aluminum filmcontaining Ti with a thickness of 300 nm, and a Ti film with a thicknessof 150 nm are continuously formed by sputtering into one is patternedinto a desired shape. Of course, the other conductive film may be used.

Further, in Embodiment 3, an ITO film with a thickness of 110 nm isformed as a pixel electrode 5063, and then subjected to patterning. Acontact is obtained by arranging the pixel electrode 5063 so as tooverlap with the connect wiring 5062 while contacting therewith.Besides, a transparent conductive film in which 2 to 20% of zinc oxideis mixed with indium oxide may be used. This pixel electrode 5063becomes an anode of an EL element (FIG. 10A).

Then, as shown in FIG. 10B, an insulating film containing silicon(silicon oxide film in Embodiment 3) is formed into a thickness of 500nm, and an opening is formed at a position corresponding to the pixelelectrode 5063 to form the third interlayer insulating film 5065. Uponthe formation of the opening, taper-shape side walls can easily beformed by using a wet etching method. If the side walls of the openingis sufficiently smooth, degradation of the EL layer caused by the stepbecomes a remarkable problem.

Then, an EL layer 5066 and a cathode (MgAg electrode) 5067 arecontinuously formed by vapor deposition without exposing them to theatmosphere. Note that the thickness of the EL layer 5066 is preferablyset as 80 to 200 nm (typically 100 to 120 nm), and the thickness of thecathode 5067 is preferably set as 180 to 300 nm (typically 200 to 250nm).

In this step, the EL layer and the cathode are sequentially formed withrespect to the pixels corresponding to a red color, a green color, and ablue color, respectively. Note that, the EL layer lacks withstandproperty against solutions, and therefore the respective colors must beformed individually without using a photolithography technology. Forthat reason, it is preferred that portions other than desired pixels aremasked using metallic masks, and the EL layer and the cathode areselectively formed only for the necessary portions.

In other words, a mask for masking all the portions except the pixelscorresponding to a red color is first set, and the EL layer emitting ared color and the cathode are selectively formed using the mask. Then, amask for masking all the portions except the pixels corresponding to agreen color is set, and the EL layer emitting a green color and thecathode are selectively formed using the mask. Succeedingly, similarly,a mask for masking all the portions except the pixels corresponding to ablue color is set, and the EL layer emitting a blue color and thecathode are selectively formed using the mask. Note that, in this case,a description is made such that a different mask is used for each case,however, the same mask may be used for all the cases.

Employed in this case is a system in which three kinds of EL elementscorresponding to RGB are formed. However, the following systems may beused: a system in which an EL element emitting a white color and a colorfilter are combined; a system in which an EL element emitting a blue orblue-green color and a fluorescing body (fluorescing color conversionlayer: CCM) are combined; and a system in which a transparent electrodeis used for a cathode (opposing electrode) and an EL elementcorresponding to the RGB is overlapped therewith.

Note that known materials may be used for the EL layer 5066. As theknown materials, organic materials are preferably used when taking adriver voltage into an account. For example, a four-layer structureconsisting of a hole injection layer, a hole transport layer, a lightemitting layer, and an electron injection layer may be used as the ELlayer.

Next, the cathode 5067 is formed using a metal mask on the pixels havingthe switching TFTs of which the gate electrodes are connected to thesame gate signal line (pixels on the same line). Note that, inEmbodiment 3, although MgAg is used as the cathode 5067, the presentinvention is not limited to this. Other known materials may be used forthe cathode 5067.

Finally, a passivation film 5068 made from a silicon nitride film isformed into a thickness of 300 nm. By forming the passivation film 5068,the EL layer 5066 can be protected from moisture, etc., and thereliability of the EL element may be enhanced.

Consequently, the EL display device with the structure as shown in FIG.10B is completed. Note that, in the manufacturing process of the ELdisplay in Embodiment 3, the source signal lines are formed from Ta andW, which are materials for forming gate electrodes, and the gate signallines are formed from Al, which is a wiring material for formingdrain/source electrode, but different materials may be used.

Incidentally, the EL display device in Embodiment 3 exhibits the veryhigh reliability and has the improved operational characteristic byproviding TFTs having the most suitable structure in not only the pixelportion but also the driver circuit portion. Further, it is alsopossible to add a metallic catalyst such as Ni in the crystallizationprocess, thereby increasing crystallinity. It therefore becomes possibleto set the driving frequency of the source signal line driver circuit to10 MHZ or higher.

First, a TFT having a structure in which hot carrier injection isreduced without decreasing the operating speed as much as possible isused as an n-channel TFT of a CMOS circuit forming the driver circuitportion. Note that the driver circuit referred to here includes circuitssuch as a shift register, a buffer, a level shifter, a latch inline-sequential drive, and a transmission gate in dot-sequential drive.

In Embodiment 3, the active layer of the n-channel TFT contains thesource region, the drain region, the LDD region overlapping with thegate electrode with the gate insulating film sandwiched therebetween(Lov region), the LDD region not overlapping with the gate electrodewith the gate insulating film sandwiched therebetween (Loff region), andthe channel forming region.

Further, there is not much need to worry about degradation due to thehot carrier injection with the p-channel TFT of the CMOS circuit, andtherefore LDD regions may not be formed in particular. It is of coursepossible to form LDD regions similar to those of the n-channel TFT, as ameasure against hot carriers.

In addition, when using a CMOS circuit in which electric current flowsin both directions in the channel forming region, namely a CMOS circuitin which the roles of the source region and the drain regioninterchange, it is preferable that LDD regions be formed on both sidesof the channel forming region of the n-channel TFT forming the CMOScircuit, sandwiching the channel forming region. A circuit such as atransmission gate used in dot-sequential drive can be given as anexample of such. Further, when a CMOS circuit in which it is necessaryto suppress the value of the off current as much as possible is used,the n-channel TFT forming the CMOS circuit preferably has an Lov region.A circuit such as the transmission gate used in dot-sequential drive canbe given as an example of such.

Note that, in practice, it is preferable to perform packaging (sealing),without exposure to the atmosphere, using a protecting film (such as alaminated film or an ultraviolet cured resin film) having good airtightproperties and little outgassing, or a transparent sealing material,after completing through the state of FIG. 10B. At this time, thereliability of the EL element is increased by making an inert atmosphereon the inside of the sealing material and by arranging a drying agent(barium oxide, for example) inside the sealing material.

Further, after the airtight properties have been increased by thepackaging process, a connector (flexible printed circuit: FPC) isattached in order to connect terminals led from the elements or circuitsformed on the substrate with external signal terminals. Then, a finishedproduct is completed. This state at which the product is ready forshipment is referred to as a display device throughout thisspecification.

Furthermore, in accordance with the process shown in Embodiment 3, thenumber of photo masks required for manufacture of a display device canbe suppressed. As a result, the process can be shortened, and thereduction of the manufacturing cost and the improvement of the yield canbe attained.

Embodiment 4

FIG. 11A is a top surface diagram of an EL display device using thedriving method of the present invention. In FIG. 11A, reference numeral4010 denotes a substrate, while reference numeral 4011 denotes a pixelportion, 4012 denotes a source signal line driver circuit, and 4013denotes a gate signal line driver circuit. The respective drivercircuits are connected to an external equipment via wirings 4014 and4016 leading to an FPC 4017.

A cover material 6000, an airtight sealing material (also referred to asa housing material) 7000, and a sealing material (a second sealingmaterial) 7001 are provided at this time so as to surround at least thepixel portion, and preferably the driver circuit and the pixel portion.

Further, FIG. 11B is a cross sectional structure of the EL displaydevice of Embodiment 4, and a driver circuit TFT (note that a CMOScircuit in which an n-channel TFT and a p-channel TFT are combined isshown in the figures here) 4022 and a pixel portion TFT 4023 (note thatonly an EL driving TFT is shown in the figures here) are formed on abase film 4010 on the substrate 4021. Known structures (top gatestructures or bottom gate structures) may be used for these TFTs.

After completing the driver circuit TFT 4022 and the pixel portion TFT4023 by using a known method of manufacturing, a pixel electrode 4027made from a transparent conducting film for electrically connecting to adrain of the pixel portion TFT 4023 is formed on an interlayerinsulating film (leveling film) 4026 made from a resin material. Acompound of indium oxide and tin oxide (referred to as ITO) and acompound of indium oxide and zinc oxide can be used as the transparentconducting film. An insulating film 4028 is formed once the pixelelectrode 4027 is formed, and an open portion is formed on the pixelelectrode 4027.

An EL layer 4029 is formed next. A lamination structure of a known ELmaterial (hole injecting layer, hole transporting layer, light emittinglayer, electron transporting layer, and electron injecting layer), or asingle layer structure, may be used for the EL layer 4029. Further,there are low molecular weight materials and high molecular weightmaterials (polymer materials) for the EL material. An evaporation methodis used when a low molecular weight material is used, but it is possibleto use a simple method such as printing or spin coating of ink-jetprinting when a high molecular weight material is used.

The EL layer is formed by evaporation using a shadow mask in Embodiment4. Color display becomes possible by forming light emitting layers (ared color light emitting layer, a green color light emitting layer, anda blue color light emitting layer) capable of emitting light atdifferent wavelength for each pixel using the shadow mask. In addition,a method of combining a color changing layer (CCM) and a color filter,and a method of combining a white color light emitting layer and a colorfilter are available, and both may be used. Of course, a single colorlight emitting electronic device can also be made.

After forming the EL layer 4029, a cathode 4030 is formed on the ELlayer. It is preferable to remove as much moisture and oxygen aspossible from the interface between the cathode 4030 and the EL layer4029. A method in which the EL layer 4029 and the cathode 4030 areformed in succession within a vacuum, or in which the EL layer 4029 isformed in an inert environment and the cathode 4030 is then formedwithout exposure to the atmosphere is therefore necessary. The abovefilm formation can be performed by using a multi-chamber method (clustertool method) film formation apparatus.

Note that a lamination structure of a LiF (lithium fluoride) film and anAl (aluminum) film is used as the cathode 4030 in Embodiment 4.Specifically, a 1 nm thick LiF (lithium fluoride) film is formed byevaporation on the EL layer 4029, and a 300 nm thick aluminum film isformed on the LiF film. An MgAg electrode, which is a known cathodematerial, may of course also be used. The cathode 4030 is then connectedto the wiring 4016 in a region denoted by reference numeral 4031. Thewiring 4016 is an electric power source supply line for applying apredetermined voltage to the cathode 4030, and is connected to the FPC4017 through a conducting paste material 4032.

The cathode 4030 and the wiring 4016 are electrically connected in theregion shown by reference numeral 4031, and therefore it is necessary toform contact holes in the interlayer insulating film 4026 and in theinsulating film 4028. These contact holes may be formed during etchingof the interlayer insulating film 4026 (when the pixel electrode contacthole is formed) and during etching of the insulating film 4028 (whenforming the open portion before forming the EL layer). Further, etchingmay also be performed together through to the interlayer insulating film4026 when etching the insulating film 4028. A contact hole having a goodshape can be formed in this case provided that the interlayer insulatingfilm 4026 and the insulating film 4028 are formed by the same resinmaterial.

A passivation film 6003, a filler material 6004 and the cover material6000 are formed covering the surface of the EL element thus formed.

In addition, the sealing material 7000 is formed on the inside of thecover material 6000 and the substrate 4010 so as to surround the ELelement portion. The airtight sealing material (the second sealingmaterial) 7001 is formed on the outside of the sealing material 7000.

The filler material 6004 functions as an adhesive for bonding the covermaterial 6000. PVC (polyvinyl chloride), epoxy resin, silicone resin,PVB (polyvinyl butyral) and EVA (ethylene vinyl acetate) can be used asthe filler material 6004. A moisture absorption effect can be maintainedif a drying agent is formed on the inside of the filler material 6004,and therefore it is preferable to do so.

Furthermore, spacers may be included within the filler material 6004.The spacers may be made from a powdered substance composed of a materialsuch as BaO, giving the spacers themselves moisture absorbency.

The passivation film 6003 can relieve the spacer pressure for cases offorming the spacers. Further, a film such as a resin film, separate fromthe passivation film 6003, may also be formed for relieving the spacerpressure.

Further, a glass plate, an aluminum plate, a stainless steel plate, anFRP (fiberglass-reinformed plastic) plate, a PVF (polyvinyl fluoride)film, a mylar film, a polyester film, and an acrylic film can be used asthe cover material 6000. Note that when using PVB or EVA as the fillermaterial 6004, it is preferable to use a sheet having a structure inwhich several 10 of μm of aluminum foil is sandwiched by a PVF film or amylar film.

Note that, depending upon the direction of light emitted from the ELelements (light emission direction), it may be necessary for the covermaterial 6000 to have light transmitting characteristics.

Further, the wiring 4016 is electrically connected to the FPC 4017through a gap between the sealing material 7000 and the airtight sealingmaterial 7001, and the substrate 4010. Note that, although the wiring4016 is explained here, the other wiring 4014 are also electricallyconnected to the FPC 4017 by passing under the sealing material 7000 andthe airtight sealing material 7001.

Note that the cover material 6000 is bonded after forming the fillermaterial 6004 in FIG. 11, and that the sealing material 7000 is attachedso as to the side surface (exposed surface) of the filler material 6004,but the filler material 6004 may also be formed after attaching thecover material 6000 and the sealing material 7000. A filler materialinjection port passing through the gap formed by the substrate 4010, thecover material 6000 and the sealing material 7000 is formed in thiscase. The gap is then placed in a vacuum state (equal to or less than10⁻² torr), and after immersing the injection port in a tank containingthe filler material, the pressure on the outside of the gap is madehigher than the pressure within the gap, and the filler material fillsthe space.

Embodiment 5

Next, an example of manufacturing the EL display device which have adifferent form from that shown in FIGS. 11A and 11B is explained usingFIGS. 12A and 12B. The explanation of the same number as FIGS. 11A and11B are omitted because they are indicated same portion.

FIG. 12A is a top view of an EL display device using the presentinvention. FIG. 12B shows a cross sectional view which is cut along theline A-A′ in FIG. 12A.

A passivation film 6003 is formed covering the surface of the EL elementthus made according to FIG. 11.

The filler material 6004 is provided and further functions as anadhesive for bonding the cover member 6000. PVC (polyvinyl chloride),epoxy resin, silicone resin, PVB (polyvinyl butyral), and EVA (ethylenevinyl acetate) can be used as the filler material 6004. If a dryingagent is formed on the inside of the filler material 6004, then it cancontinue to maintain a moisture absorbing effect, which is preferable.

Further, spacers may be contained within the filler material 6004. Thespacers may be a powdered substance such as BaO, giving the spacersthemselves the ability to absorb moisture.

When using spacers, the passivation film 6003 can relieve the spacerpressure. Further, a film such as a resin film can be formed separatelyfrom the passivation film to relieve the spacer pressure.

Furthermore, a glass plate, an aluminum plate, a stainless steel plate,an FRP (fiberglass-reinforced plastics) plate, a PVF (polyvinylfluoride) film, a Mylar film, a polyester film, and an acrylic film canbe used as the cover material 6000. Note that if PVB or EVA is used asthe filler material 6004, it is preferable to use a sheet with astructure in which several tens of μm of aluminum foil is sandwiched bya PVF film or a Mylar film.

However, depending upon the light emission direction from the EL element(the light radiation direction), it is necessary for the cover material6000 to have light transmitting characteristics.

Next, the cover material 6000 is bonded by using the filler material6004. Thereafter, a frame member 6001 is attached so as to cover sidesurfaces (exposed surfaces) formed by the filler material 6004. Theframe member 6001 is bonded by a sealing material 6002 (functioning asan adhesive). Preferably, a photo-setting resin is used as sealingmaterial 6002. However, a thermosetting resin may be used if the heatresistance of the EL layer is high enough to allow use of such a resin.It is desirable that the sealing material 6002 has such properties as toinhibit permeation of moisture and oxygen as effectively as possible. Adesiccant may be mixed in the sealing material 6002.

Further, the wiring 4016 is electrically connected to the FPC 4017through a gap between the sealing material 6002 and the substrate 4010.Note that although an explanation of the wiring 4016 has been made here,the wiring 4014 is also electrically connected to the FPC 4017 bysimilarly passing underneath the sealing material 6002.

In FIG. 12, the cover material 6000 is bonded after forming the fillermaterial 6004, and the frame material 6001 is attached so as to coverthe lateral surfaces (exposed surfaces) of the filler material 6004, butthe filler material 6004 may also be formed after attaching the covermaterial 6000 and the frame material 6001. In this case, an injectionopening of the filler material is formed through a gap formed by thesubstrate 4010, the cover material 6000, and the frame material 6001.The gap is set into a vacuum state (a pressure equal to or less than10⁻² Torr), and after immersing the injection opening in the tankholding the filler material, the air pressure outside of the gap is madehigher than the air pressure within the gap, and the filler materialfills the gap.

Embodiment 6

Here, FIG. 13 illustrates a further detailed structure in cross sectionof a pixel portion of an EL display device. In FIG. 13, a switching TFT4502 provided on a substrate 4501 is an n-channel type TFT formed by aknown method. In the present embodiment, the switching TFT 4502 is of adouble gate structure with gate electrodes 39 a and 39 b. By adoptingthe double gate structure, two TFTs are substantially connected inseries, and thus, there is an advantage that an off current value can bedecreased. It is to be noted that, though the double gate structure isadopted in the present embodiment, a single gate structure, a triplegate structure, or a multiple gate structure having more than threegates may also be adopted. Further, a p-channel type TFT formed by aknown method may also be used.

In the present embodiment, an EL driving TFT 4503 is an n-channel typeTFT formed by a known method. A gate electrode 37 of the EL driving TFT4503 is electrically connected to a drain wiring 35 of the switching TFT4502 via a wiring 36.

Since the EL driving TFT is an element for controlling the amount ofelectric current through the EL element, a lot of electric currentpasses through it, and thus, it is highly liable to deterioration due toheat or due to hot carrier. Therefore, a structure, in which an LDDregion is provided at the side of a drain of the EL driving TFT so as tooverlap the gate electrode through a gate insulating film, is quiteeffective.

Further, a single gate structure with one gate electrode 37 of the ELdriving TFT 4503 is shown in the figures in this embodiment, but amulti-gate structure in which a plurality of TFTs are connected inseries may also be used. In addition, a structure in which a pluralityof TFTs are connected in parallel, with partition into a plurality ofchannel forming regions, and which can perform radiation of heat withhigh efficiency, may also be used.

Though the top gate TFT is used in this embodiment, the bottom gate TFTcan also be used.

Further, a source wiring 40 is connected to a power supply line (notillustrated), and constant voltage is always applied to the sourcewiring 40.

A first passivation film 41 is formed on the switching TFT 4502 and theEL driving TFT 4503, and a leveling film 42 comprising an insulatingresin film is formed on the first passivation film 41. It is extremelyimportant to level the step due to the TFTs using the leveling film 42.An EL layer formed later is extremely thin, and there are cases in whichdefective light emissions occur. Therefore, to form the EL layer withits surface which is as level as possible, it is preferable to performleveling before forming a pixel electrode.

Furthermore, reference numeral 43 denotes a pixel electrode (a cathodeof an EL element in this case) made from a conductive film with highreflectivity, and is electrically connected to a drain region of the ELdriving TFT 4503. It is preferable to use a low resistance conductivefilm, such as an aluminum alloy film, a copper alloy film, and a silveralloy film, or a laminate of such films. Of course, a laminationstructure with another conductive film may also be used.

In addition, a light emitting layer 45 is formed in a groove(corresponding to a pixel) formed by banks 44 a and 44 b formed ofinsulating films (preferably resins). Note that only one pixel is shownin the figure here, but the light emitting layer may correspond to eachof the colors R (red), G (green), and B (blue). A π conjugate polymermaterial is used as an organic EL material. Polyparaphenylene vinylenes(PPVs), polyvinyl carbazoles (PVKs), and polyfluoranes can be given astypical polymer materials.

Note that there are several types of PPV organic EL materials, andmaterials recorded in Shenk, H., Becker, H., Gelsen, O., Kluge, E.,Kreuder, W., and Spreitzer, H., “Polymers for Light Emitting Diodes”,Euro Display Proceedings, 1999, pp. 33-7, and in Japanese PatentApplication Laid-open No. Hei 10-92567, for example, may be used.

As specific light emitting layers, cyano-polyphenylene vinylene may beused as a red light emitting layer, polyphenylene vinylene may be usedas a green light emitting layer, and polyphenylene vinylene orpolyalkylphenylene may be used as a blue light emitting layer. The filmthickness may be between 30 and 150 nm (preferably between 40 and 100nm).

However, the above example is merely one example of the organic ELmaterials which can be used as light emitting layers, and it is notnecessary to limit use to these materials. An EL layer may be formed byfreely combining light emitting layers, electron transport layers, andelectron injection layers.

For example, the present embodiment shows an example of using a polymermaterial as a light emitting layer, but a low molecular weight organicEL material may also be used. Further, it is possible to use inorganicmaterials such as silicon carbide, as an electron transport layer or anelectron injection layer. Known materials can be used for these organicEL materials and inorganic materials.

An EL layer with a laminate structure, in which a hole injection layer46 made of PEDOT (polythiophene) or PAni (polyaniline) is formed on thelight emitting layer 45, is used in the present embodiment. An anode 47is then formed on the hole injection layer 46 of a transparentconductive film. The light generated in the light emitting layer 45 isradiated toward the upper surface (the opposite direction to thesubstrate 4501 where TFT is formed) in the present embodiment, andtherefore the anode must have a conductive property and be formed of amaterial with a property of being transparent to light. A compound ofindium oxide and tin oxide, or a compound of indium oxide and zinc oxidecan be used as the transparent conductive film. However, because it isformed after forming the low heat resistance light emitting and holeinjection layers, it is preferable to use a material which can bedeposited at as low a temperature as possible.

An EL element 4505 is complete at the point where the anode 47 isformed. Note that what is called the EL element 4505 here is formed bythe pixel electrode (anode) 43, the light emitting layer 45, the holeinjection layer 46, and the anode 47. The pixel electrode 43 is nearlyequal in area to the pixel, and consequently the entire pixel functionsas an EL element. Therefore, the light emitting efficiency is extremelyhigh, and a bright image display becomes possible.

In addition, a second passivation film 48 is then formed on the anode 47in the present embodiment. It is preferable to use a silicon nitridefilm or an oxidized silicon nitride film as the second passivation film48. The purpose of this is the isolation of the EL element from theoutside, and this is meaningful in preventing degradation due tooxidation of the organic EL material, and in controlling gaseous emittedfrom the organic EL material. The reliability of the EL display can thusbe raised.

The EL display device of the present invention has a pixel portion madefrom pixels structured as in FIG. 13, and has a switching TFT with asufficiently low off current value, and a current control TFT which isstrong with respect to hot carrier injection. An EL device with highreliability, and in which good image display is possible, can thereforebe obtained.

Embodiment 7

In this embodiment, there will be described a structure in which thestructure of the EL element 4505 is reversed in the pixel portionillustrated in Embodiment 6. Explanation will be given with reference toFIG. 14. Note that, since the points of difference from the structureshown in FIG. 13 lie only in the EL element and the driver TFT, theother explanation shall be omitted from description.

Referring to FIG. 14, an EL driving TFT 4503 is formed using thep-channel TFT manufactured by known method.

In this embodiment, a transparent conductive film is employed as a pixelelectrode (anode) 50. Concretely, the conductive film is made of acompound of indium oxide and zinc oxide. Of course, a conductive filmmade of a compound of indium oxide and tin oxide may well be employed.

Besides, after banks 51 a and 51 of an insulating film have been formed,a light emitting layer 52 made of polyvinylcarbazole is formed on thebasis of the application of a solution. An electron injection layer 53made of potassium acetylacetonate (expressed as acacK), and a cathode 54made of an aluminum alloy are formed thereon. In this case, the cathode54 functions also as a passivation film. Thus, an EL element 4701 isformed.

In the case of this embodiment, light generated in the light emittinglayer 52 is radiated toward a substrate 4501 formed with TFTs asindicated by an arrow.

Embodiment 8

The following description of Embodiment 8 is the construction of thesource signal line driver circuit.

FIG. 6 is a circuit diagram showing the source signal line drivercircuit. A shift register 8801, a latch (A) 8802 and a latch (B) 8803are disposed as shown in FIG. 6. In Embodiment 8, the latch (A) 8802 andthe latch (B) 8803 are disposed to correspond to four source signallines S_a to S_d. Embodiment 8 is not provided with a level shifter forvarying the amplitude width of the voltage of a signal, the level shiftmay be disposed as required.

A clock signal CLK, a clock signal CLKB which has an opposite polarityto the clock signal CLK, a start pulse signal SP and a driving-directionswitching signal SL/R are inputted to the shifter register 8801 via therespective lines shown in FIG. 6. A digital signal VD which is inputtedfrom the outside is divided into four signals, and the four signals areinputted to the latch (A) 8802 via the respective lines shown in FIG. 6.A latch signal S_LAT and a signal S_LATb which has an opposite polarityto the latch signal S_LAT are inputted to the latch (B) 8803 via therespective lines shown in FIG. 6.

When the signal output from the shifter register 8801 is inputted to thelatch (A) 8802, the latch (A) 8802 obtains four signals at the same timefrom the four divided digital signals VD. In response to the latchsignals S_LAT and S_LATb, the latch (B) 8803 latches the digital signalsVD and output them to the source signal lines S_a to S_d.

In the above description of Embodiment 8, the method of simultaneouslysampling signals corresponding to four source signal lines with the useof four divided video signals. However, in general, n-number of divideddigital signals may be used to simultaneously sample signalscorresponding to n-number of source signal lines.

The detailed construction of the latch (A) 8802 will be described belowwith illustrative reference to a part 8804 of the latch (A) 8802 whichcorresponds to the source signal line S_a. The part 8804 of the latch(A) 8802 has two clocked inverters and two inverters.

FIG. 7 is a top plan view showing a part 8804 of the latch (A) 8802.Reference numerals 831 a and 831 b denote active layers of TFTs whichform one of the inverters of the part 8804 of the latch (A) 8802, andreference numeral 836 denotes a gate electrode which is common to theTFTs which form the one of the inverters. Reference numerals 832 a and832 b denote active layers of TFTs which form the other of the invertersof the part 8804 of the latch (A) 8802, and reference numerals 837 a and837 b denote gate electrodes which are provided over the active layers832 a and 832 b, respectively. Incidentally, the gate electrodes 837 aand 837 b are electrically connected to each other.

Reference numerals 833 a and 833 b denote active layers of TFTs whichform one of the clocked inverters of the part 8804 of the latch (A)8802. Gate electrodes 838 a and 838 b are provided over the active layer833 a so as to constitute a double-gate structure. The gate electrode838 b and a gate electrode 839 are provided over the active layer 833 bso as to constitute a double-gate structure.

Reference numerals 834 a and 834 b denote active layers of TFTs whichform the other of the clocked inverters of the part 8804 of the latch(A) 8802. The gate electrode 839 and a gate electrode 840 are providedover the active layer 834 a so as to constitute a double-gate structure.The gate electrode 840 and a gate electrode 841 are provided over theactive layer 834 b so as to constitute a double-gate structure.

Embodiment 9

Embodiment 9 will be described below with reference to FIGS. 15A and 15Bwhich show a fabricated example of an EL display device using thedriving method according to the invention. FIG. 15A is a top plan viewshowing the state in which EL elements formed on an active matrixsubstrate are sealed. Sections 6801, 6802 and 6803, each of which isshown by dashed lines, are a source signal line driver circuit, a gatesignal line driver circuit and a pixel portion, respectively. Sections6804, 6805 and 6806 are a cover material, a first sealing material and asecond sealing material, respectively. A filler 6807 (refer to FIG. 15B)is provided in the inside portion surrounded by the first sealingmaterial 6805 between the cover material 6804 and the active matrixsubstrate.

Reference numeral 6808 denotes connecting lines to transmit inputsignals to the source signal line driver circuit 6801, the gate signalline driver circuit 6802 and the pixel portion 6803, and the connectinglines 6808 receive video signals and clock signals from an FPC (flexibleprinted circuit) 6809 which serves as a connecting terminal to externalequipment.

FIG. 15B is a cross-sectional view taken along line A-A′ of FIG. 15A. InFIGS. 15A and 15B, the same reference numerals are used to denote thesame portions.

As shown in FIG. 15B, the pixel portion 6803 and the source signal linedriver circuit 6801 are formed on a substrate 6800, and the pixelportion 6803 is made of plural pixels each including a TFT 6851 forcontrolling current to flow through an EL element (hereinafter referredto as the EL driving TFT 6851), a pixel electrode 6852 electricallyconnected to a drain region of the EL driving TFT 6851 and the like. InEmbodiment 9, the EL driving TFT 6851 is a p-channel type TFT. Thesource signal line driver circuit 6801 is formed of a CMOS circuit inwhich an N-channel type TFT 6853 and a P-channel type TFT 6854 arecomplementarily combined with each other.

Each pixel has a color filter (R) 6855, a color filter (G) 6856 or acolor filter (B) (not shown) under its pixel electrode. The color filter(R) is a color filter which extracts red light, the color filter (G) isa color filter which extracts green light, and the color filter (B) is acolor filter which extracts blue light. The color filter (R) 6855 isprovided in a pixel which emits red, the color filter (G) 6856 isprovided in a pixel which emits green, and the color filter (B) isprovided in a pixel which emits blue.

The first advantage of the case where these color filters are providedis that the color purity of each emitted color is improved. For example,red light is emitted from the EL element of each pixel which emits red(toward the pixel electrode in the present embodiment), and the purityof red can be improved by passing the red light through the color filterwhich extracts red light. The other green light and blue light are alsosubjected to similar processing.

In a conventional structure which does not use color filters, there mayoccur the problem that visible light which enters from the outside of anEL display device excites the emitting layers of its EL elements and nodesired colors can be obtained. However, if color filters are disposedas in the case of Embodiment 9, light with particular wavelength is onlyallowed to enter the EL elements. That is to say, it is possible toprevent the problem that the EL elements are excited by external light.

Incidentally, although structures provided with color filters haveheretofore been proposed, white-emitting EL elements have been used insuch structures. In this case, light with the other wavelengths is cutoff to extract red light, so that a lowering of luminance is incurred.However, in Embodiment 9, since red light emitted from the EL elementsis passed through color filters which extract red light, a lowering ofluminance is prevented from being incurred.

The pixel electrode 6852 is formed of a transparent conductive film, andfunctions as the anode of the EL element. Insulating films 6857 areformed at both ends of the pixel electrode 6852, and in addition, anemitting layer 6858 which emits red light and an emitting layer 6859which emits green light are formed. Incidentally, although not shown, anemitting layer which emits blue light is provided in an adjacent pixel,whereby color display is provided by the pixels which individuallycorrespond to red, green and blue. Of course, the pixels comprisingblue-emitting layers are provided with color filters which extract bluelight.

Not only an organic material but also an inorganic material may be usedas an EL material. In addition, a stacked structure, in which anelectron injection layer, an electron transport layer, a hole transportlayer and a hole injection layer are combined, may be adopted.

A cathode 6860 of the EL element is formed of a conductive film withlight-shielding characteristics, on each of the emitting layers. Thiscathode 6860 is common to all the pixels, and is electrically connectedto the FPC 6809 via connecting lines 6808.

Then, the first sealing material 6805 is formed with a dispenser or thelike, and spacers (not shown) are scattered and the cover material 6804is stuck. Then, the area which is surrounded by the active matrixsubstrate 6800, the cover material 6804 and the first sealing material6805 is filled with the filler 6807 by a vacuum injection method.

In addition, in Embodiment 9, barium oxide is previously added to thefiller 6807 as a hygroscopic material 6861. Incidentally, in Embodiment9, the filler 6807 is a filler containing a hygroscopic material, butthe hygroscopic material may also be sealed in the filler in the stateof being dispersed in massive form. Although not shown, a hygroscopicmaterial may also be used as the material of spacers.

Then, after the filler 6807 has been cured by irradiation of ultravioletrays or by heating, an opening (not shown) formed in the first sealingmaterial 6805 is closed. After the openings of the first sealingmaterial 6805 have been closed, the connecting lines 6808 and the FPC6809 are electrically connected to each other by the use of a conductivematerial 6862. In addition, a second sealing material 6806 is formed tocover the exposed portion of the first sealing material 6805 and a partof the FPC 6809. The second sealing material 6806 may use the samematerial as the first sealing material 6805.

By sealing the EL elements with filler 6807 with the use of theabove-described method, it is possible to completely isolate the ELelements from the outside, whereby a substance which promotes oxidationof an organic material, such as water or oxygen, can be prevented frompenetrating from the outside. Accordingly, it is possible to fabricate ahighly reliable EL display device.

Embodiment 10

In the description of Embodiment 10, an example will be described, inwhich the direction of irradiation of light emitted from EL elements andthe arrangement of color filters differ in the EL display deviceaccording to Embodiment 9. The following description uses FIG. 16, butthe pixel portion shown in FIG. 16 is the same in basic structure asthat shown in FIG. 15B, and the modified portions of the constructionshown in FIG. 16 are denoted by different reference numerals.

As shown in FIG. 15B, a pixel portion 6901 is formed of plural pixelseach including a TFT 6902 for controlling current to flow through the ELelement (hereinafter referred to as the EL driving TFT 6902), a pixelelectrode 6903 electrically connected to a drain region of the TFT 6902and the like.

In Embodiment 10, an n-channel type TFT is used as the EL driving TFT6902 in the pixel portion 6901. The pixel electrode 6903 is electricallyconnected to the drain of the EL driving TFT 6902, and is formed of aconductive film with light-shielding characteristics. In Embodiment 10,the pixel electrode 6903 serves as a cathode of the EL element.

A transparent conductive film 6904 which is common to all the pixels isformed on the light emitting layer 6858 which emits red light and thelight emitting layer 6859 which emits green light. The transparentconductive film 6904 serves as an anode of the EL element.

In addition, Embodiment 10 has the feature that a color filter (R) 6905,a color filter (G) 6906 and a color filter (B) (not shown) are formed onthe cover material 6804. In the case of the structure of the EL elementaccording to Embodiment 10, the direction of irradiation of lightemitted from the EL layer is toward the cover material 6804, whereby thecolor filters can be disposed in the path of light in the structureshown in FIG. 16.

In the case of forming the color filter (R) 6905, the color filter (G)6906 and the color filter (B) (not shown) on the cover material 6804,there is the advantage that it is possible to reduce the number of thesteps required to fabricate the active matrix substrate and it ispossible to realize an improvement in yield factor and throughput.

Embodiment 11

In the EL display device using the driving method according to thepresent invention, the material for the EL layers of EL elements is notlimited to organic EL materials, and may also use inorganic ELmaterials. However, since the current inorganic EL materials need veryhigh driving voltages, it is necessary to use TFTs which have breakdownvoltage characteristics against such driving voltages.

In addition, if inorganic EL materials with lower driving voltage aredeveloped in the future, such inorganic EL materials can be applied tothe present invention.

Embodiment 12

In the EL display device using the driving method according to thepresent invention, organic materials to be used for the EL layer may below molecular weight organic materials or polymeric (high molecularweight) organic materials. Materials such as Alq₃(tris-8-quinolinolato-aluminum) and TPD (triphenylamine derivative) areknown as the low molecular weight organic materials. As the polymericorganic materials, π-conjugated polymeric materials are used.Representative examples are PPV (polyphenylene vinylene), PVK(poly(vinylcarbazole) and polycarbonate.

The polymeric (high molecular weight) organic materials can be formed bya simple thin-film deposition method such as a spin-coating method (alsocalled a solution-applying method), a dipping method, a dispensingmethod, a printing method, or an ink-jet method, and have higher heatresistance then molecular weight organic materials.

In the EL element of an EL display device, if the EL layer of the ELelement has an electron transport layer and a hole transport layer, theelectron transport layer and the hole transport layer may be formed ofan amorphous semiconductor of an inorganic material such as amorphous Sior amorphous Si_(1-x)C_(x).

In the amorphous semiconductor, a large number of trap levels arepresent, and a large number of interfacial levels are formed at theinterface where the amorphous semiconductor is in contact with anotherlayer. Accordingly, the EL elements can emit light at low voltage, and afar higher luminance can be realized.

Dopants (impurities) may also be added to an organic EL layer to changethe color of light to be emitted from the organic EL layer. The dopantsare DCM 1, nile red, rubrene coumarin 6, TPB, quinacridone and the like.

Embodiment 13

This embodiment will be described on electronic devices incorporated anEL display device using the driving method of the invention.

As these electronic devices, there can be enumerated a video camera, adigital camera, a head-mountable display (a goggle type display), a gamemachine, a car navigation, a personal computer, and a mobile informationterminal (e.g., a mobile computer, a mobile telephone or an electronicbook), as shown in FIGS. 18A to 18E.

FIG. 18A shows a personal computer including a body 2001, a casing 2002,a display portion 2003 and a keyboard 2004. The EL display device usinga driving method of the present invention can be used as the displayportion 2003 of the personal computer.

FIG. 18B shows a video camera including a body 2101, a display portion2102, a voice input unit 2103, manipulation switches 2104, a battery2105 and an image receiving unit 2106. The EL display device using adriving method of the present invention can be used as the displayportion 2102 of the video camera.

FIG. 18C shows one portion (i.e., a right-hand side) of a head-mounteddisplay including a body 2301, a signal cable 2302, a head fixing band2303, a display unit 2304, an optical system 2305 and a display portion2306. The EL display device using a driving method of the presentinvention can be used the display portion 2306 of the head-mounteddisplay.

FIG. 18D shows an image reproducing device (e.g., a DVD reproducingdevice) provided with a recording medium. The image reproducing deviceincludes a body 2401, a recording medium (CD, LD or DVD and the like)2402, manipulation switches 2403 and display units (a) 2404 and (b)2405. The display portion 2404 (a) displays a image information and thedisplay portion (b) 2405 displays character information. The EL displaydevice using a driving method of the present invention can be used thedisplay portion (a) 2404 and (b) 2405. Here, this device is enabled toCD reproduction device and the game device as the recording medium.

FIG. 18E shows a mobile computer including a body 2501, a camera portion2502, a image receiving unit 2503, an operation switch 2504 and adisplay portion 2505. The EL display device using a driving method ofthe present invention can be used as the display portion 2505 of themobile computer.

As has been described hereinbefore, the invention can have an extremelywide range of applications and can be applied to electronic devices ofany fields. On the other hand, the electronic device of this embodimentcan be realized by using a construction of any of the combinations ofEmbodiments 1 to 12.

In the related art gray scale display method for active matrix type ELdisplay devices, there has been the problem that the amount of currentwhich flows in the EL elements of an EL display device becomesnon-uniform due to the unevenness of the characteristics of the TFTs ofthe pixel portion of the EL display device or due to variations in theenvironmental temperature during the use of the EL display device, sothat unevenness occurs in the luminance display of the EL displaydevice.

However, owing to the above-described construction, the invention makesit possible to keep a current which flows in each of the EL elements ofthe pixel portion, constant with respect to variations in temperature,thereby suppressing the unevenness of display. Accordingly, it ispossible to provide a driving method for an EL display device capable ofhigh-quality display.

1. A display device comprising: pixels each comprising: a first thinfilm transistor; a second thin film transistor; a resistor; and an ELelement, wherein: a gate of the first thin film transistor iselectrically connected to a gate line, one of a source and a drain ofthe first thin film transistor is directly connected to a source line,the other of the source and the drain of the first thin film transistoris directly connected to a gate of the second thin film transistor, oneof a source and a drain of the second thin film transistor iselectrically connected to a first terminal of the resistor, a secondterminal of the resistor is electrically connected to a power supplyline, the other of the source and the drain of the second thin filmtransistor is electrically connected to the EL element, the second thinfilm transistor is operated in a saturation region, the pixels aredriven by a time gray scale method, in which gray scales are representedby a cumulation of periods per frame period during which each of the ELelements emits light, and the EL element overlaps with the first thinfilm transistor and the second thin film transistor.
 2. A display deviceaccording to claim 1, wherein a ratio of a gate width of the second thinfilm transistor to a gate length of the second thin film transistor isless than
 1. 3. A display device according to claim 1, wherein a ratioof a gate width of the second thin film transistor to a gate length ofthe second thin film transistor is less than 0.5.
 4. A display deviceaccording to claim 1, further comprising a substrate, wherein: the firstthin film transistor and the second thin film transistor are providedover the substrate, and the EL element is provided over the first thinfilm transistor and the second thin film transistor.
 5. A display deviceaccording to claim 1, further comprising a substrate, wherein: the firstthin film transistor and the second thin film transistor are providedover the substrate, the EL element is provided over the first thin filmtransistor and the second thin film transistor, and the EL element emitslight toward the substrate.
 6. A display device according to claim 1,further comprising a first substrate and a second substrate, wherein:the first thin film transistor and the second thin film transistor areprovided over the first substrate, the EL element is provided over thefirst thin film transistor and the second thin film transistor, thesecond substrate is provided over the EL element, and the EL elementemits light toward the second substrate.
 7. A display device accordingto claim 1, further comprising a driver circuit, wherein: the drivercircuit includes at least an n-channel thin film transistor and ap-channel thin film transistor.
 8. An electronic device using thedisplay device according to claim
 1. 9. A display device comprising:pixels each comprising: a first thin film transistor; a second thin filmtransistor; a resistor; and an EL element, wherein: a gate of the firstthin film transistor is electrically connected to a gate line, one of asource and a drain of the first thin film transistor is directlyconnected to a source line, the other of the source and the drain of thefirst thin film transistor is directly connected to a gate of the secondthin film transistor, one of a source and a drain of the second thinfilm transistor is electrically connected to a first terminal of theresistor, a second terminal of the resistor is electrically connected toa power supply line, the other of the source and the drain of the secondthin film transistor is electrically connected to the EL element, thesecond thin film transistor is operated in a saturation region in orderto keep a current supplied from the second thin film transistor constantwith respect to temperature variations, the pixels are driven by a timegray scale method, in which gray scales are represented by a cumulationof periods per frame period during which each of the EL elements emitslight, and the EL element overlaps with the first thin film transistorand the second thin film transistor.
 10. A display device according toclaim 9, wherein a ratio of a gate width of the second thin filmtransistor to a gate length of the second thin film transistor is lessthan
 1. 11. A display device according to claim 9, wherein a ratio of agate width of the second thin film transistor to a gate length of thesecond thin film transistor is less than 0.5.
 12. A display deviceaccording to claim 9, further comprising a substrate, wherein: the firstthin film transistor and the second thin film transistor are providedover the substrate, and the EL element is provided over the first thinfilm transistor and the second thin film transistor.
 13. A displaydevice according to claim 9, further comprising a substrate, wherein:the first thin film transistor and the second thin film transistor areprovided over the substrate, the EL element is provided over the firstthin film transistor and the second thin film transistor, and the ELelement emits light toward the substrate.
 14. A display device accordingto claim 9, further comprising a first substrate and a second substrate,wherein: the first thin film transistor and the second thin filmtransistor are provided over the first substrate, the EL element isprovided over the first thin film transistor and the second thin filmtransistor, the second substrate is provided over the EL element, andthe EL element emits light toward the second substrate.
 15. A displaydevice according to claim 9, further comprising a driver circuit,wherein: the driver circuit includes at least a n-channel thin filmtransistor and a p-channel thin film transistor.
 16. An electronicdevice using the display device according to claim
 9. 17. A displaydevice comprising: pixels each comprising a first thin film transistor;a second thin film transistor; a resistor; and an EL element, wherein: agate of the first thin film transistor is electrically connected to agate line, one of a source and a drain of the first thin film transistoris directly connected to a source line, the other of the source and thedrain of the first thin film transistor is directly connected to a gateof the second thin film transistor, one of a source and a drain of thesecond thin film transistor is electrically connected to a firstterminal of the resistor, a second terminal of the resistor iselectrically connected to a power supply line, the other of the sourceand the drain of the second thin film transistor is electricallyconnected to the EL element, and an absolute value of a gate voltage ofthe second thin film transistor is not greater than an absolute value ofa voltage between the other of the source and the drain of the secondthin film transistor and the second terminal of the resistor, the pixelsare driven by a time gray scale method, in which gray scales arerepresented by a cumulation of periods per frame period during whicheach of the EL elements emits light, and the EL element overlaps withthe first thin film transistor and the second thin film transistor. 18.A display device according to claim 17, wherein a ratio of a gate widthof the second thin film transistor to a gate length of the second thinfilm transistor is less than
 1. 19. A display device according to claim17, wherein a ratio of a gate width of the second thin film transistorto a gate length of the second thin film transistor is less than 0.5.20. A display device according to claim 17, further comprising asubstrate, wherein: the first thin film transistor and the second thinfilm transistor are provided over the substrate, and the EL element isprovided over the first thin film transistor and the second thin filmtransistor.
 21. A display device according to claim 17, furthercomprising a substrate, wherein: the first thin film transistor and thesecond thin film transistor are provided over the substrate, the ELelement is provided over the first thin film transistor and the secondthin film transistor, and the EL element emits light toward thesubstrate.
 22. A display device according to claim 17, furthercomprising a first substrate and a second substrate, wherein: the firstthin film transistor and the second thin film transistor are providedover the first substrate, the EL element is provided over the first thinfilm transistor and the second thin film transistor, the secondsubstrate is provided over the EL element, and the EL element emitslight toward the second substrate.
 23. A display device according toclaim 17, further comprising a driver circuit, wherein: the drivercircuit includes at least an n-channel thin film transistor and ap-channel thin film transistor.
 24. An electronic device using thedisplay device according to claim 17.